This invention relates generally to an opto-electrical apparatus, such as, a liquid crystal display device and, more particularly, relates to opto-electrical apparatus utilizing nonlinear devices, such as, MIM devices, to drive pixel electrodes such as in a liquid crystal display panel.
In conventional liquid crystal display panels employed in opto-electrical apparatus, a plurality of pixel electrodes are arranged in a matrix on an appropriate substrate and with a companion substrate retain therebetween a liquid crystal layer. Each pixel electrode is driven through nonlinear devices exhibiting nonlinear current/voltage characteristics. FIG. 7 shows a portion of a substrate, commonly referred to as the lower substrate, including a conventional nonlinear device, S, of the MIM type. FIG. 8 is a cross sectional view taken along the line 8--8 of FIG. 7. In reference to FIGS. 7 and 8, substrate 1 is generally made of transparent glass. A conductor 2 is formed on substrate 1, such as, for example, comprising tantalum (Ta). Next, an insulator 3 comprising tantalum oxide (TaO.sub.x) is formed on conductor 2, i.e., over its top surface 3B and along its sides or side edges 3A and 3C. Next, a conductor 4 is formed on a portion of the surface of insulator 3 including one side edge 3A of insulator 3. Second conductor 4 may be comprised of, for example, chrome (Cr). Lastly, pixel electrode 5 is formed on a portion of the surface of glass substrate 1 and a portion of second conductor 4, as shown in both FIGS. 7 and 8. This arrangement of MIM devices, S, and associated pixel electrodes 5 are fabricated over the entire surface of substrate 1 in a matrix configuration. In the example of FIGS. 7 and 8, first conductor 2 is formed to have inter-pixel connection for signal input to each of the pixel electrodes 5.
Thus, lateral MIM device S comprises first and second conductors 2, 4, with insulator 3 formed between these conductors, and together function as a nonlinear passive device for driving pixel electrodes 5. As shown in FIG. 8, in the case of nonlinear MIM device, S, insulator 3 comprises a thicker insulator portion 3B covering the top surface of conductor 2 and a thinner insulator portion 3A covering a side edge of conductor 2. In this manner, insulator portion 3B functions as a barrier layer with high electrical resistance while thinner insulator portion 3A extends over a smaller area of conductor, i.e., its side edge, and will breakdown at higher voltages to permit the flow of current, as is characteristic of a nonlinear device. Only insulator portion 3A on side region 2A of conductor 2 is employed as breakdown region of nonlinear MIM device, S.
As indicated above, by employing thin side insulator 3A along edge 2A of conductor 2 to function as a nonlinear device, the surface area of the nonlinear device can be fabricated to be a very small dimension. As a result, this lateral MIM structure is highly effective for developing high density, high yield LCD panels and other LC devices employing this small lateral MIM device as a nonlinear device for driving the pixel electrodes.
FIG. 9 illustrates the step-by-step process utilized in fabricating lateral MIM device, S. First, as shown in FIG. 9A, a conductive film or layer 2', such as, Ta, is formed on the top surface of transparent substrate 1, which comprises glass. Next, as illustrated in FIG. 9B, conductive film 2' is selectively formed to a predetermined shape by means of photo etching to produce elongated first conductor 2 on substrate 1. Next, as shown in FIG. 9C, an insulating layer or film 3', such as, TaO.sub.x, is formed over conductor 2 and on substrate 1.
Next, the selective formation of thick barrier layer 3B is carried out as shown in FIG. 9D. The entire construction of insulator 3 is formed by employing photo etching. Since first conductor 2 is not a transparent film and insulating film 3' is transparent, photosensitive etching can be utilized with exposure thereof through the back surface of substrate 1 wherein conductor 2 functions as a photo resist mask. First, the surface of film 3' is coated with a positive photosensitive resist, i.e., with a photoresist in which photo exposed areas are removed. Exposure of the photosensitive resist takes place from the back surface of transparent substrate 1 and, thereafter, is developed so that the remaining photosensitive resist will be only that formed on the surfaces of conductor 2. After this, barrier layer 3B is formed, as indicated in FIG. 9D, by etching and removing exposed portions of insulating layer 3'. Barrier layer 3B on the top surface of insulator 3 creates a barrier to current flow relative to first conductor 2.
Next, as shown in FIG. 9E, insulator portion 3A are formed by means of anodic oxidation. An example of the process for performing anodic oxidation is described in U.S. application Ser. No. 07/880,120 filed May 7, 1992 in the name of Takashi Nakazawa and entitled "THIN FILM TRANSISTOR AND METHOD 0F MANUFACTURE". However, other processes may be utilized to form insulator portions 3A, such as, by CVD, sputtering and thermal oxidation. Insulator portion 3A becomes the operative insulator portion for nonlinear MIM device S which is formed on the side of first conductor 2 The formation of this oxide is followed by the formation of second conductors 4 and pixel electrodes 5, as shown in FIG. 9F, which are selectively made in their predetermined shapes employing photo etching. The lateral overlapping portion of first conductor 2, thin insulator 3A and second conductor 4, formed on the side edge 2A of first conductor 2 together constitute lateral MIM device S.
However, with the formation of a lateral MIM device S, there is also undesirably formed a parasitic or secondary or dummy MIM device S' constituted by first conductor 2, thicker barrier layer 3B, atop conductor 2, and portion 4A of second conductor 4 formed on the top of barrier layer 3B, in spite of the greater thickness of barrier layer 3B. This secondary MIM device S' provides a detrimental influence on the desired characteristics of primary lateral MIM device S. As a result, to eliminate or otherwise significantly reduce this detrimental influence, it is necessary to make the resistance component of the secondary MIM device S' significantly large and the capacitance component of the secondary MIM device significantly small. In order to accomplish this, it is necessary that the thickness of barrier layer 3B be made as thick as possible. However, if barrier layer 3B is made too large in thickness, the fabrication process will become too difficult to handle. For example, the thicker the device height, the longer is the process time to fabricate the device. If the MIM device height is made to be twice that of the conventional MIM height, for example, the time required for fabrication will be twice as long as the conventional device. Therefore, in the case of line fabrication, it is much more efficient and less costly to reduce the fabrication time such as by one half.
Furthermore, there are other problems created in increasing the thickness of the insulator, such as, corresponding increase in the occurrence of defects, e.g., broken connection lines; difficulty in auto photo-alignment; difficulty in focusing; and an increase in the ease of undesirably increasing deviations of pattern sizes during the photo etching process. This latter problem is of particular sensitivity because since the size of the MIM devices in an opto-electrical apparatus determine the contrast ratio, differences in resulting patterns and, therefore, sizes of MIM devices can be fatal so that the resulting yield in the production of such apparatus depends greatly on the uniformity of the size of the MIM devices employed in the apparatus.
In general, in the past, an oxide film having a film thickness of 3,000 .ANG. has been employed for barrier layer 3B. However, for the reasons previously mentioned, it is impractical, if not impossible, to make the barrier film thicker, for example, twice as thick, e.g., 6,000 .ANG..
It is an object of this invention to decrease the capacitance and increase the resistance in the primary MIM device while substantially eliminating the affect of the secondary MIM device.
It is another object of this invention to improve the operational performance of a lateral MIM device as utilized in opto-electrical apparatus without requiring or utilizing a thicker insulating barrier layer.
It is another object of this invention to enhance the utilization of MIM nonlinear devices through the overall functioning of a dual nonlinear structure effective in the reduction of capacitance.